Current and voltage references are building blocks used in virtually every mixed-signal system. There are a variety of methods for implementing voltage or current references, ranging from the comparison of bias voltages across simple semiconductor devices to the quantum tunneling of electric charge on floating-gate devices.
One method for providing voltage and current references uses the silicon energy bandgap. In bandgap reference circuits, the reference current or voltage is derived from two p-n junctions operated at different current densities, each having a different forward bias voltage drop. The voltage difference between forward voltage drops is applied across a resistor to generate a proportional to absolute temperature (PTAT) current, which is further converted into a (PTAT) voltage. The PTAT voltage can then be added to a complementary to absolute temperature (CTAT) voltage derived from another p-n junction. The voltage can then be applied to a reference resistor to produce a thermally compensated reference current.
However, recent technological advances use low-voltage complementary metal oxide semiconductor (CMOS) circuits designed to reduce power consumption and to extend battery life of portable devices, operating at lower supply voltages. Thus, voltage head-room has become increasingly limited, making it difficult to use conventional bandgap reference circuits in such low-power applications.
In the following description, the use of the same reference numerals in different drawings indicates similar or identical items.